1. Field of the Invention
The invention is generally related to construction of offshore structures and more particularly to the transport and installation of heavy decks offshore.
2. General Background
An extensive number of offshore structures have been placed on the sea floor around the world to produce oil and gas reserves commonly found on the Outer Continental Shelf. The platforms combine the substructure, which extends from the sea floor to an elevation above the sea level, and the deck structure, which houses all the equipment necessary for the operation of the platform. During the construction of the offshore platform a certain amount of offshore assembly is generally required. In order to minimize the offshore construction costs, the platforms are often subdivided into a few large components.
The substructure is normally fabricated as a single unit onshore, skidded onto a transport and/or launch barge, towed to the site, launched or lifted off from the barge, and placed on the sea floor by ballasting. Piling is driven into the sea floor through pile sleeves or other features in the substructure after which the piling is secured to the driven piling by grouting, welding, or other mechanical methods.
The function of the substructure is to support the superstructure, or the deck. The deck contains the equipment necessary for the operation of the platform. Ideally, the deck is built as a complete unit onshore with all of its equipment installed and tested. The deck is then skidded onto a transport barge, towed to offshore installation site, lifted by a high capacity floating crane, and mounted on a previously installed substructure. If the deck is too heavy to be lifted by the available barge-mounted floating crane, then it has to be installed by one of two methods described below.
The most common method is to subdivide the deck into smaller units that can be lifted by the floating crane. The separate units are fabricated onshore and the equipment in each of the units is tested to the extend possible. The units are skidded onto transport barges, towed to the offshore installation site, lifted by a floating crane and placed on the previously installed substructure, or on previously installed deck units. There are significant disadvantages to this method. It takes longer to lift and place several units instead of a single unit. The individual modules have to be able to match with each other. An extra amount of structure must be constructed. More significantly, it takes extra time to make necessary connections between the units and test all the equipment prior to production
A method exists that permits the deck to be fabricated as a complete unit onshore with all its equipment installed and tested before it is skidded onto the transportation barge. For this method to be successful, the substructure is specially designed with a large open area in its central region near the waterline. The transport barge, with a deck onboard, is floated into this central region and moored with the deck in the mating position. In the mating position, legs extending downward from the deck are directly above legs projecting upward from the substructure on either side of the barge. The deck is mated to the substructure by ballasting the barge downward until it rests on the substructure legs.
This method permits the deck to be fabricated as a complete unit onshore and eliminates the use of floating cranes offshore. However, this method puts severe limits on the operation. A deck that can not be lifted by a floating crane is fairly massive. Such a deck will require a wide transportation barge so that it will be stable against roll. A wide barge requires an even larger opening in the central region of the substructure in which to fit the transport barge. Thus, this renders this method suitable for only wide decks and substructures. Also, the opening in the substructure causes a large span of deck between the support points, so the structural efficiency gained in the complete deck construction may be lost. The depth of the barge controls the height at which the deck is set. Most significantly, the time required to ballast the barge during set down operation is significant, requiring a long weather window for transfer operations. Also, the ballasting rate is such that there exists a possibility of damage to the deck due to the barge""s heave motions.
The invention addresses the above need. What is provided is a method to transport a deck at sea and install the deck on an offshore substructure. The deck is completely fabricated on a set of deep girders. The deck and deep girders are skidded onto two pontoons. The deck and deep girders are tied down together. A set of jacking units is installed into the deep girders prior to sail out. The complete assembly is towed to the installation site. At the site, the tie downs are removed. The girders are jacked up and the pontoons simultaneously ballasted down until the assembly is supported on the pontoons. The deck is lowered onto the substructure and the pontoons released.